autotrophic denitratation on sulfur

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Nitrates elimination by autotrophic denitratation on sulfur
By Christophe Soler, Translated by Nicolas Will
I really would like to stress the fact that the use of a denitratation system on a reef aquarium shouldn't be necessary, a balanced and well established tank being, generally, enough for nitrates elimination. This technique is then, in my opinion, more adapted to fish-only aquariums, but it can be used on reef aquariums where nitrates are a permanent worry.
Nitrates accumulation is without any doubt one of the most encountered problem in aquaria, whether using salt or fresh water. An efficient and original solution was applied by the biologist of Paris' Aquarium du Musée des Arts Africains et Océaniens (MAAO, Aquariums of the Museum of African and Oceanian Arts) and of Saint-Malo's Grand Aquarium.
Classic denitrators use the metabolism of anaerobic bacteria to reduce nitrates into gaseous nitrogen. This anaerobic bacteria need for this task an addition of carbonated stuff (sugars, alcools) and a great hypoxy (lack of oxygen)
Michel Hignette, Benoît Lamort, Marc Langouet, Sebastien Leroy and Guy Martin had the idea to use certain bacteria' property to remove nitrates from water by using sulfur as an energy source. Those bacteria (Thiobacillus denitrificans) are in fact capable of removing nitrates from water when they are in a slightly hypoxic place and if a native sulfur substrate is available.

System's principle
The aquarium's water is forced through a tube containing chunks of native sulfur (around 5-mm). The bacteria develop extremely rapidly on this sulfured substrate. They will reduce nitrates into gaseous nitrogen, and oxidize sulfur into non-toxic sulfates.

Use
The cartridges containing the sulfur can simply be made out of PVC tubes of 50, 75, 100-mm or more, depending on the aquarium's volume. 6 to 8 liters of sulfur need necessary to efficiently remove nitrates from a 600 liters tank. Sulfur is in the form of chunks (around 5-mm) or beads. The water circulating through the cartridge is taken out of filter return derivation. At installation time, the rate of water going through must be very low (set using a Mohr valve). The system starts up very rapidly (in 48 hours at 26 degrees centigrade and just measurements of nitrites concentration of the output water must be made. They will give high results. The rate of water flowing through the cartridge must then be increased until the nitrite concentration falls to zero. The level of nitrates must also be lower then the one of input water.

System's advantages
- Very short startup delay, around 48 hours, compared to the 1 month startup delay of a classic denitrator (heterotrophic denitratation)
- Total autonomy of the system once started. No addition of nutritive carbonated solution (like glucose, methanol or other), the bacteria get their energy from the sulfur that they oxidize into sulfates
- The necessary hypoxy to this process is far less strict than the on necessary for a classic denitrator, the flow rate in the cartridge can be more important, thus excluding any risk of producing undesired compounds like sulfured hydrogen
- Finally, this system seems ore efficient than heterotrofic denitrators when nitrates concentration is low (<20 mg/l).
System's drawbacks
- A release of sulfates in a quantity more or less equal to the quantity of used nitrates (1.1 mol of sulfates produced for 1 mol of nitrates reduced). However, no toxicity was reported in the numerous tanks equipped to this day in the MAAO and in Saint-Malo. One shouldnít forget that natural seawater sulfates concentration is high (nearly 2700 mg/l). This parameter should be monitored.
- The water's acidity lowers the buffering power. This acidity can be used by coupling the sulfur cartridge to a cartridge filled with crushed coral or aragonite. The water's acidity will dissolve the content and produce calcium and carbonates. This drawback becomes a serious advantage!

Bibliography:
Hignette, M., Lamort, B., Langouet, M., Leroy, S. et Martin, G. 1997. Elimination des nitrates par filtration biologique autotrophe sur soufre en aquariologie marine. Mém. Inst. Océano. P. Ricard pp 7-13.